1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP sensor with low current-induced noise.
2. Background of the Invention
One type of conventional magnetoresistive sensor used as the read head in magnetic recording disk drives is a “spin-valve” (SV) sensor. A SV magnetoresistive sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.
In a magnetic recording disk drive SV read sensor or head, the stack of layers are located in the read “gap” between magnetic shields. The magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the SV is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor.
CPP-SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January 2002. Another type of CPP sensor is a magnetic tunnel junction (MTJ) sensor in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a MTJ magnetoresistive read head the spacer layer is electrically insulating and is typically alumina (Al2O3 ) or MgO; in a CPP-SV magnetoresistive read head the spacer layer is electrically conductive and is typically copper.
CPP sensors are susceptible to current-induced noise and instability. The spin-polarized current flows perpendicularly through the ferromagnetic layers and produces a spin transfer torque on the local magnetization. This can produce continuous gyrations of the magnetization, resulting in substantial low-frequency magnetic noise if the sense current is above a certain level. This effect is described by J.-G. Zhu et al., “Spin transfer induced noise in CPP read heads,” IEEE TRANSACTIONS ON MAGNETICS, Vol. 40, pp. 182-188, January 2004. In a related paper it was suggested, but not demonstrated, that the sensitivity to spin-torque-induced instability of the free layer could be reduced by use of a dual spin-valve. (J.-G. Zhu et al., “Current induced noise in CPP spin valves,” IEEE TRANSACTIONS ON MAGNETICS, Vol. 40, No. 4, pp. 2323-2325, July 2004). However, a dual spin-valve requires a second spacer layer on the free layer and a second pinned layer on the second spacer layer, and thus results in a larger shield-to-shield read gap distance, which lowers sensor resolution.
What is needed is a CPP sensor that produces minimal current-induced noise without loss of magnetoresistance or sensor resolution.